1. Field of the Invention
The present invention relates to an iron-based alloy containing chromium, manganese, cobalt, carbon, silicon and nickel wherein the nickel is added in a range facilitating the addition of amounts of silicon which promote cavitation erosion resistance without unacceptable brittleness.
2. Brief Description of the Prior Art
Turbine blades in a hydroelectric generator undergo cavitation erosion. Cavitation erosion results from pressure differences in the water close to the surface of the blade. When the local pressure falls below the vapor pressure of the water, a cavity or vapor bubble develops in the liquid. When the pressure rises again above that of the vapor, the vapor bubble abruptly collapses sending a shock wave to the metal surface. Eventually, the metal in the blades fatigues, forms cracks and sections spall off. As cavitation erosion progresses, the rotor becomes unbalanced and the whole hydroelectric generator may begin to vibrate. To fix the problem, the rotor must be pulled from the generator and the damaged blades resurfaced by welding them with an alloy provided as a wire ductile enough to conform to the damaged blade. The repair is then ground to profile.
There are many weldable iron-cobalt-chromium alloys with excellent cavitation erosion resistance but not with the unique combination of features provided by the alloy described in this disclosure, including a balance of cavitation erosion resistance, ductility, hardness and cost. For example, STELLITE.RTM. 21 is a cavitation erosion resistant alloy used as reference standard against which other alloys are measured. STELLITE 21 typically contains, in weight percent, 27 chromium, 5.5 molybdenum, 2 nickel, 1.5 silicon and 0.25 carbon with the balance being cobalt and is expensive because of the high cobalt content. STELLITE is a registered trademark of Stoody Deloro Stellite, Inc. Another alloy sold by Stoody Deloro Stellite is TRISTELLE.TM. TS-2. This alloy is described in U.S. Pat. No. 4,487,630 to Crook et al. and contains, in weight percent, 35 chromium, 12 cobalt, 10 nickel, 4.9 silicon and 2 carbon with the balance being iron. According to U.S. Pat. No. 4,487,630, levels of nickel above 5% by weight are required to promote an austenitic structure. TRISTELLE TS-2 is more resistant to cavitation than STELLITE 21 and is less expensive because it contains less cobalt; however, TRISTELLE TS-2 is brittle, making it crack sensitive when welded. It is also very hard, making it difficult to grind to a smooth profile when it is used to resurface turbine blades. Other weldable, cavitation erosion resistant alloys include HQ 913.RTM., an alloy described in U.S. Pat. Nos. 4,588,440 and 4,751,046, assigned to Hydro Quebec of Montreal, Canada. HQ 913 is another registered trademark of Stoody Deloro Stellite, Inc. HQ 913 typically contains, in weight percent, 17.0 chromium, 10.0 manganese, 9.5 cobalt, 2.8 silicon, 0.25 nickel, 0.20 nitrogen and 0.17 carbon with the balance being iron. The amount of silicon in HQ 913 is restricted by the amount of nickel which, in turn, is limited by phase requirements.
Each of the above-mentioned iron-cobalt-chromium alloys differs in some subtle way from the others, providing a different alloy suited for certain specific uses. Such differences include, for example, a new range of an effective element or a critical ratio of certain elements already specified with valuable advances in alloy development being made in small unexpected, but effective increments.